Voice over Internet Protocol - VOIP

Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of voice conversations over the Internet or through any other IP-based network, VoIP technology users a software which is used to conduct telephone-like voice conversations across the internet. VOIP phone service is often cheaper than traditional phone service, and is becoming more popular for both business and personal calls

Lowratevoip is one of several Internet telephony services (also called VoIP-service) of Betamax GmbH & Co. KG in Germany. Formerly Lowratevoip was a service of Swiss based Finarea SA. Lowratevoip uses a client software with proprietary protocol, but can also be accessed via SIP-protocol. Lowratevoip features low call rates. Subscribers can mutually call for free, and call for free to fixed lines in selected countries worldwide (0 pennies per minute and no start tariff) for a limited number of days after payments to Lowratevoip

There are alot of VOIP-services, click on the link "Comparison of VoIP software" to view them in a table

Building your own VOIP
There are serveral ways which you can use to build your own VOIP network, some of which you need a big invistement, and others are cheap; one example method which is commonly used nawadays is Asterisk

Asterisk® is a complete IP PBX in software. Released as open source under the GNU General Public License (GPL), Asterisk runs on a wide variety of operating systems including Linux, Mac OS X, OpenBSD, FreeBSD and Sun Solaris and provides all of the features you would expect from a PBX including many advanced features that are often associated with high end (and high cost) proprietary PBXs. Asterisk® supports Voice over IP in many protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware